The 'Manage Variables' option is not available or is inaccessible in the HTML5 version of flowgear. https://developers.flowgear.net/kb/Node:Variable_Bar
^ It says to right click the Node Header and choose 'Manage Variables' but right clicking the Node does not do anything. The only option available when clicking the gear icon on then node is the option to delete it and the option to check help which links to the page above.